1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer which uses an electrophotographic system or electrostatic recording system.
2. Description of the Related Art
In an image forming apparatus of the electrophotographic system, density unevenness (known as banding) occurs in the sub scanning direction of an image due to the periodic rotation unevenness of a photosensitive drum, intermediate transfer belt driving roller, development roller itself, motors and gears which drive them, or the like. More specifically, as rotation unevenness occurs in a photosensitive drum, the laser write position periodically varies. In addition, when rotation unevenness occurs in the driving roller of the intermediate transfer belt, the transfer position periodically varies. Furthermore, when rotation unevenness occurs in the development roller, the development state periodically varies. Variations in position lead to variations in scanning line interval (so-called pitch errors), which appear as density unevenness. In addition, variations in development are variations in main scanning line density, and appear as density unevenness. These periodic variations appear as banding on an image, resulting in a deterioration in print quality.
To solve this problem of banding, Japanese Patent Laid-Open No. 2007-108246 has proposed a technique of correcting an image signal so as to cancel banding, that is, a so-called banding image correction method.
Conceivable banding image correction methods include a density correction method of correcting the tones of an image in opposite directions so as to cancel density unevenness caused by the above position offsets and variations in development state and a position correction method of moving scanning positions on an image signal in opposite directions so as to cancel the above position offsets. A conceivable position correction method is a method of performing pseudo correction for less than one line by using multilevel values for PWM (Pulse Width Modulation) in addition to line-based correction.
Japanese Patent Laid-Open No. 2007-108246 has proposed a method of solving density unevenness in the sub scanning direction by the above density correction. More specifically, first of all, a density sensor measures the density unevenness of banding caused by an image forming apparatus. This method then predicts density unevenness during image formation from the measured density unevenness, and corrects an image signal so as to cancel the density unevenness. When, for example, density correction is performed before halftone processing, the corrected state may not be stored depending on the subsequent halftone processing, resulting in a failure to reduce banding. In addition, performing the above position correction will make the above problem more noticeable. It is therefore necessary to perform banding image correction after halftone processing. On the other hand, in order to reduce the amount of data transferred, save memory, and reduce the cost of a PWM circuit, the number of bits of an image signal after halftone processing is preferably smaller than that before halftone processing.
If, however, the number of bits of an image signal is small, since the resolution is not high enough to reflect a correction amount, the correction accuracy becomes insufficient. This rather worsens the image quality. For example, horizontal streaks appear due to correction errors. FIG. 24 shows examples of pitch errors as correction results obtained when the above position correction is performed for banding caused by pitch errors of a given period after halftone processing, and the resultant correction amounts are quantized into 8-bit data each and 4-bit data each. Obviously, the 8-bit quantization (solid line) for the original pitch errors (thick line) suppresses the pitch errors to almost 0, whereas the 4-bit quantization (broken line) produce sudden large pitch errors, which cause a deterioration in image quality in the form of sudden streaks. That is, the smaller the number of bits expressing a correction amount, the larger a quantization error.